# Certificate SD fields

> 1SD/CI/tolerance/EU are not interchangeable

Source: https://geodb.io/guide/topic/qaqc_certificate_sd_fields/ · Category: QAQC
A CRM certificate carries several dispersion numbers, and **they are not
interchangeable**. An OREAS certificate has a Table 1 (`Certified Value |
1SD | 95% Confidence | 95% Tolerance`) and separate Performance Gates
(`1SD | 2SD | 3SD | …`). Two of those are **explicitly disqualified from
QC use**, in the certificate's own words:

> "95% Confidence Limits **should not be used as control limits** for
> laboratory performance." — "tolerance limits pertain to the homogeneity
> of the CRM only and should not be used as control limits."

The **1SD** is the correct control-limit field. Confidence intervals and
expanded uncertainty **shrink as more labs certify the CRM** — so using
them gives a *better-characterised* CRM *tighter* false gates. The error
is silent and always in the failing direction.

## Vendor traps to know by name

- **OREAS (Feb 2026 onward):** Table 1 dropped 1SD for *95% Expanded
  Uncertainty* (half-width ≈ 1×SD). Read it as 2SD and every gate is ~2×
  too tight.
- **CDN (current):** the column **titled "Two Standard Deviations"
  contains 1SD** (verified arithmetically). Older single-line CDN certs
  genuinely are 2SD. Same vendor, opposite conventions.
- **Rocklabs:** sanctions **no** certificate field as a control limit at
  all — build limits from your own lab's data.
- **AMIS:** the headline ± is the control-chart SD, not the between-lab SD.

And two structural traps: **certified values are per-method** (OREAS 254c
certifies Au four different ways — analyte alone is not a unique key), and
**`IND`/`nr` must be parsed per-cell** — a row can have a valid SD next to
an `IND` tolerance.

## In geoDB

geoDB stores certified values **per method** — the QC engine matches a
standard's certified value to the analytical method on the certificate,
which is exactly the discipline the per-method trap requires. Standards
imported from the built-in OREAS/CDN library carry vetted 1SD values. When
you enter a standard by hand from a certificate PDF, this topic is the
checklist: find the true 1SD, not the headline ±.